According to PVMBG a three-minute-long phreatic eruption at Dempo began at 1651 on 9 November, and generated a dense ash plume that rose 4.2 km (13,800 ft) a.s.l., about 1 km above the crater rim, and drifted S. The Alert Level remained at 2 (on a scale of 1-4).

Our most recent report on Dempo (BGVN 34:01) discussed a phreatic eruption on 1 January 2009. This eruption prompted Indonesia's Center of Volcanology and Geological Hazard Mitigation (CVGHM) to raise the alert level from 1 (normal) to 2 (alert) on a scale of 1-4. A few months later, on 23 March 2009, the CVGHM lowered Dempo's Alert Level to 1, based on visual observations of the crater lake during 5-6 January and 2-4 March, and decreased seismicity since the 1 January phreatic eruption.

The name Dempo applies to both the larger complex and to a peak that sits adjacent to a neighboring peak called Marapi. The latter volcano name applies to several different volcanoes in Indonesia and is easily confused with the very prominent volcano Merapi (central Java). The Marapi cone in the Dempo complex contains a ~ 400 m diameter crater lake, the source of both the 2006 and 2009 eruptions.

The remainder of this report discusses the phreatic eruption that occurred in September 2006. This eruption had not been previously discussed in the Bulletin, and CVGHM reporting on the subject has recently come to our attention.

In the month before this eruption, teams from the CVGHM had visited several times. On 13 August 2006, a team prepared a map of Dempo and reported that the condition of the water in Marapi's crater lake was normal and clear or slightly blue in color, with no bubbling. On 4 September 2006, a team climbed to the peak of Dempo and reported that activity was normal other than some bubbling at the E edge of the crater lake.

2006 phreatic eruption. On 25 September 2006, a phreatic eruption occurred that expelled water from Marapi's crater lake and propelled mud onto the area around the peak up to a radius of 300 m.

On the next day, the inspector for Dempo, Mr. Mulyadi, accompanied by six friends, inspected the volcano and its lake. According to Mulyadi's team, the lake water was bubbling and had changed to a grayish color. Acrid sulfurosu emissions were accompanied by a hissing sound. The NW crater wall was covered by mud from the eruption. Many of the phreatic deposits around the crater lake were only about 0.5 cm thick, although in several other places they were thicker (figures 1-4). A visit on 4 October 2006 found the deposits not yet eroded, owing to a lack of rain since the eruption.

Figure 1. Photo of the plume resulting from Dempo's phreatic eruption on 25 September 2006. This is one of a set of multiple photos taken of a rising plume. Photo by Fredy, a local resident. Courtesy of CVGHM.

Figure 2. (top) Pre-eruption and (bottom) post-eruption scenes from the saddle between the cones of Dempo and Marapi looking upslope towards Marapi (which contains the source vent in a steep-sided crater not visible from this perspective). The shots were taken on 7 and 26 September 2009, respectively. Freshly deposited mud and evidence of ejected crater-lake water in the foreground (bottom) represents distal deposits from the 25 September eruption; note peson at right for scale. Large blocks were not from this eruption. Courtesy of the Dempo inspection team, CVGHM.

Figure 3. Pre- and post-eruption photos looking into Marapi crater at Dempo, taken on 7 (top) and 26 (bottom) September 2006. The lake is on the order of 400 m across. The bottom photo portrays the crater's mud-covered walls and sediment-covered lake. Comparison of both photos indicates that after the eruption the lake surface had dropped, consistent with discharge of water and mud. Camera look-direction unstated. Courtesy of the Dempo inspection team, CVGHM.

Figure 4. Post-eruption conditions on Dempo's Marapi cone seen from a point a few meters back from the crater rim. Widespread gray-to-brown mud covered the rim and upper crater, creating a desolate scene. The large angular blocks on the rim were placed there in previous events, not the 25 September 2009 eruption. Unstated direction; photographers shadow and gear for scale. Courtesy of the Dempo inspection team, CVGHM.

Weekly Reports - Index

According to PVMBG a three-minute-long phreatic eruption at Dempo began at 1651 on 9 November, and generated a dense ash plume that rose 4.2 km (13,800 ft) a.s.l., about 1 km above the crater rim, and drifted S. The Alert Level remained at 2 (on a scale of 1-4).

Observers at the PVMBG Dempo observation post reported that during January and February no plumes rose from Dempo's crater, and during 1 March-4 April diffuse white plumes rose no higher than 50 m above the crater. Seismicity increased from 21 March to 4 April. The Alert Level was raised to 2 (on a scale from 1-4) on 5 April. Visitors and residents were advised not to approach the craters within 3 km.

Observers at the PVMBG Dempo observation post reported that during 1 June-9 September no plumes rose from Dempo's crater and seismicity was low. On 10 September the Alert Level was lowered to 1 (on a scale from 1-4). Visitors and residents were advised not to approach the craters.

Observers at the PVMBG Dempo observation post reported that during 0730-0900 on 27 April diffuse gray-white plumes rose 50 m above Dempo crater. Seismicity had increased during April as compared to the previous month. On 29 April the Alert Level was raised to 2 (on a scale from 1-4). Visitors and residents were advised not to go within a 1-km radius of the summit.

On 23 March, CVGHM lowered the Alert Level for Dempo from 2 to 1 (on a scale of 1-4) based on visual observations of the crater lake during 5-6 January and 2-4 March, and decreased seismicity since a phreatic eruption on 1 January. Visitors and residents were advised not to go within a 1-km radius of the summit.

CVGHM reported that on 1 January, a phreatic eruption from Dempo resulted in a strong sulfur odor and "ash rain" that was noted as far as about 10 km from the summit. Fog prevented direct observations of the summit. The Alert Level was raised to 2 (on a scale from 1-4). Visitors and residents were advised not to go within a 2-km radius of the summit.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

At 1810 on 18 September an earthquake was felt in an area around Dempo. The Volcanological Survey of Indonesia (VSI) reported its Modified Mercalli (MM) Intensity as MM IV. The following day three more earthquakes were felt: the first event took place at 0900 (MM II), the next at 1155 (MM I), and the last at 1800 (MM I). Recorded seismicity during these days indicated 30 events/day with amplitudes of 20-30 mm and arrival time differences (S-P) of 2-3 seconds on average. Volcanologists took this to mean that the earthquakes were local tectonic events. No visible signs of changed activity were apparent at the crater. The VSI took the precaution of increasing the condition of alert to Level II.

According to a report by Indonesia's Center of Volcanology and Geological Hazard Mitigation (CVGHM), Dempo had a phreatic eruption on 1 January 2009. The event resulted in a strong sulfur odor and an ash (or cinder) rain that was noted as far as ~ 10 km from the summit. During 27 December 2008 to 1 January 2009, fog prevented direct observations of the summit; clouds over the summit area are a common occurrence. The one available photo of the eruption showed a whitish-colored plume rising perhaps a few kilometers over the summit.

On 1 January authorities raised the alert status from 1 (normal) to 2 (alert, "Waspada" in Indonesian), on a scale that ranges from 1-4. Visitors and residents were advised not to go within a 2-km radius of the summit. A 2 January report from CVGHM noted that both local volcanic and regional tectonic earthquakes were recorded during 27 December 2008 through 1 January 2009 (table 1).

Table 1. Highlights of Dempo seismicity recorded during 27 December 2008 to 1 January 2009. Courtesy of CVGHM.

Date

Seismicity

27 Dec 2008

One local volcanic (A-type), and two tectonic.

28 Dec 2008

One local volcanic (A-type), and one air blast event.

29 Dec 2008

One local volcanic, five air blasts, and two tectonic.

30 Dec 2008

One shallow local volcanic, one air blast, one low-frequency, and one tectonic.

31 Dec 2008

One local volcanic, one air blast, and two tectonic not at the volcano.

01 Jan 2009

One local volcanic related to the explosive eruption.

As of early 2009, no thermal anomalies had been measured over Dempo by the MODVOLC infrared satellite system for at least the last 5 years. ASTER images from 2001-2007 show extensive cloud cover.

Our most recent report on Dempo (BGVN 34:01) discussed a phreatic eruption on 1 January 2009. This eruption prompted Indonesia's Center of Volcanology and Geological Hazard Mitigation (CVGHM) to raise the alert level from 1 (normal) to 2 (alert) on a scale of 1-4. A few months later, on 23 March 2009, the CVGHM lowered Dempo's Alert Level to 1, based on visual observations of the crater lake during 5-6 January and 2-4 March, and decreased seismicity since the 1 January phreatic eruption.

The name Dempo applies to both the larger complex and to a peak that sits adjacent to a neighboring peak called Marapi. The latter volcano name applies to several different volcanoes in Indonesia and is easily confused with the very prominent volcano Merapi (central Java). The Marapi cone in the Dempo complex contains a ~ 400 m diameter crater lake, the source of both the 2006 and 2009 eruptions.

The remainder of this report discusses the phreatic eruption that occurred in September 2006. This eruption had not been previously discussed in the Bulletin, and CVGHM reporting on the subject has recently come to our attention.

In the month before this eruption, teams from the CVGHM had visited several times. On 13 August 2006, a team prepared a map of Dempo and reported that the condition of the water in Marapi's crater lake was normal and clear or slightly blue in color, with no bubbling. On 4 September 2006, a team climbed to the peak of Dempo and reported that activity was normal other than some bubbling at the E edge of the crater lake.

2006 phreatic eruption. On 25 September 2006, a phreatic eruption occurred that expelled water from Marapi's crater lake and propelled mud onto the area around the peak up to a radius of 300 m.

On the next day, the inspector for Dempo, Mr. Mulyadi, accompanied by six friends, inspected the volcano and its lake. According to Mulyadi's team, the lake water was bubbling and had changed to a grayish color. Acrid sulfurosu emissions were accompanied by a hissing sound. The NW crater wall was covered by mud from the eruption. Many of the phreatic deposits around the crater lake were only about 0.5 cm thick, although in several other places they were thicker (figures 1-4). A visit on 4 October 2006 found the deposits not yet eroded, owing to a lack of rain since the eruption.

Figure 1. Photo of the plume resulting from Dempo's phreatic eruption on 25 September 2006. This is one of a set of multiple photos taken of a rising plume. Photo by Fredy, a local resident. Courtesy of CVGHM.

Figure 2. (top) Pre-eruption and (bottom) post-eruption scenes from the saddle between the cones of Dempo and Marapi looking upslope towards Marapi (which contains the source vent in a steep-sided crater not visible from this perspective). The shots were taken on 7 and 26 September 2009, respectively. Freshly deposited mud and evidence of ejected crater-lake water in the foreground (bottom) represents distal deposits from the 25 September eruption; note peson at right for scale. Large blocks were not from this eruption. Courtesy of the Dempo inspection team, CVGHM.

Figure 3. Pre- and post-eruption photos looking into Marapi crater at Dempo, taken on 7 (top) and 26 (bottom) September 2006. The lake is on the order of 400 m across. The bottom photo portrays the crater's mud-covered walls and sediment-covered lake. Comparison of both photos indicates that after the eruption the lake surface had dropped, consistent with discharge of water and mud. Camera look-direction unstated. Courtesy of the Dempo inspection team, CVGHM.

Figure 4. Post-eruption conditions on Dempo's Marapi cone seen from a point a few meters back from the crater rim. Widespread gray-to-brown mud covered the rim and upper crater, creating a desolate scene. The large angular blocks on the rim were placed there in previous events, not the 25 September 2009 eruption. Unstated direction; photographers shadow and gear for scale. Courtesy of the Dempo inspection team, CVGHM.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Marapi, Gunung

Stratovolcano

3173 m

4° 2' 0" S

103° 8' 0" E

Basic Data

Volcano Number

Last Known Eruption

Elevation

LatitudeLongitude

261230

2017 CE

3173 m / 10407 ft

4.03°S
103.13°E

Volcano Types

Stratovolcano(es)
Caldera

Rock Types

MajorAndesite / Basaltic Andesite

Tectonic Setting

Subduction zoneContinental crust (> 25 km)

Population

Within 5 kmWithin 10 kmWithin 30 kmWithin 100 km

124
9,613
96,058
2,307,584

Geological Summary

Dempo is a prominent 3173-m-high stratovolcano that rises above the Pasumah Plain of SE Sumatra. The andesitic Dempo volcanic complex has two main peaks, Gunung Dempo and Gunung Marapi, constructed near the SE rim of a 3 x 5 km caldera breached to the north. The one called Dempo is slightly lower, with an elevation of 3049 m and lies at the SE end of the summit complex. The taller Marapi cone, with a summit elevation 3173 m, was constructed within a crater cutting the older Gunung Dempo edifice. Remnants of 7 craters are found at or near the summit of the complex, with volcanism migrating to the WNW with time. The large, 800 x 1100 m wide historically active summit crater cuts the NW side of Gunung Marapi (not to be confused with Marapi volcano 500 km to the NW in Sumatra) and contains a 400-m-wide lake located at the far NW end of the crater complex. Historical eruptions have been restricted to small-to-moderate explosive activity that produced ashfall near the volcano.

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography.

The youngest crater of Dempo volcano, at the WNW end of a series of six partially overlapping craters cutting the summit, contains a 400-m-wide crater lake that has been the source of frequent phreatic eruptions during historical time. A seventh smaller crater is located on the upper north flank. Dempo is one of Sumatra's most active volcanoes, frequently producing small-to-moderate explosive eruptions.

Photo by Ruska Hadian, 1989 (Volcanological Survey of Indonesia).

The crater wall of Kawah Merapi, the youngest and NW-most of the craters cutting the summit of Dempo volcano, is capped by thin lava flows; a crater lake is seen at the lower right in this view from the NE. Gunung Merapi, the high point of the summit region of Dempo volcano, occurs at the extreme left at the SE part of the crater rim.

Photo by Ruska Hadian, 1989 (Volcanological Survey of Indonesia).

The crater wall above the historically active crater lake of Kawah Merapi at the summit of Dempo volcano shows a series of alternating lava flows and pyroclastic deposits.

Photo by Ruska Hadian, 1989 (Volcanological Survey of Indonesia).

A scientist from the Volcanological Survey of Indonesia investigates rocks in the summit crater of Dempo volcano. The crater of Kawah Merapi on Dempo volcano truncates bedded pyroclastic material that is overlain by a series of light-colored lava flows.

Photo by R. Whandyo, 1992 (Volcanological Survey of Indonesia).

An aerial view from the north shows the summit crater of Dempo volcano at the upper left. The steep-walled crater contains a lake, out of sight in this view behind the NW crater rim. The crater lake has been the source of frequent phreatic eruptions during historical time.

Anonymous photo, 1991.

Gunung Dempo, one of the most active volcanoes of Sumatra, has a conical profile when viewed from the village of Pageralam below the eastern flank of the volcano. The summit contains nested craters on its NW side.

Photo by Sumarma Hamidi, 1972 (Volcanological Survey of Indonesia).

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Using infrared satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data, scientists at the Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, developed an automated system called MODVOLC to map thermal hot-spots in near real time. For each MODIS image, the algorithm automatically scans each 1 km pixel within it to check for high-temperature hot-spots. When one is found the date, time, location, and intensity are recorded. MODIS looks at every square km of the Earth every 48 hours, once during the day and once during the night, and the presence of two MODIS sensors in space allows at least four hot-spot observations every two days. Each day updated global maps are compiled to display the locations of all hot spots detected in the previous 24 hours. There is a drop-down list with volcano names which allow users to 'zoom-in' and examine the distribution of hot-spots at a variety of spatial scales.

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